In industrial measurements technology, especially also in connection with the control and monitoring of automated manufacturing processes, for ascertaining characteristic measured variables of fluids, for example, liquids and/or gases, flowing in a process line, for example, a pipeline, often such measuring systems are used, which, by means of a measuring transducer of the vibration type and transmitter electronics connected thereto and most often accommodated in a separate, electronics housing, induce reaction forces—for example, Coriolis forces—in the flowing fluid, and produce, recurringlly derived from these, measurement values correspondingly representing the at least one measured variable, for example, a mass flow rate, a density, a viscosity or some other process parameter. Such measuring systems—often formed by means of an in-line measuring device in compact construction with integrated measuring transducer, such as, for instance, a Coriolis mass flow meter—are long since known and have proven themselves in industrial use. Examples of such measuring systems having a measuring transducer of the vibration type, or also individual components thereof, are described e.g. in EP-A 317 340, JP-A 8-136311, JP-A 9-015015, US-A 2007/0119264, US-A 2007/0119265, US-A 2007/0151370, US-A 2007/0151371, US-A 2007/0186685, US-A 2008/0034893, US-A 2008/0141789, U.S. Pat. Nos. 4,680,974, 4,738,144, 4,777,833, 4,801,897, 4,823,614, 4,879,911, 5,009,109, 5,024,104, 5,050,439, 5,291,792, 5,359,881, 5,398,554, 5,476,013, 5,531,126, 5,602,345, 5,691,485, 5,734,112, 5,796,010, 5,796,011, 5,796,012, 5,804,741, 5,861,561, 5,869,770, 5,945,609, 5,979,246, 6,047,457, 6,092,429, 6,073,495, 6,311,136, 6,223,605, 6,330,832, 6,397,685, 6,513,393, 6,557,422, 6,651,513, 6,666,098, 6,691,583, 6,840,109, 6,868,740, 6,883,387, 7,017,424, 7,040,179, 7,073,396, 7,077,014, 7,080,564, 7,134,348, 7,216,550, 7,299,699, 7,305,892, 7,360,451, 7,392,709, 7,406,878, WO-A 00/14 485, WO-A 01/02 816, WO-A 2004/072588, WO-A 2008/013545, WO-A 2008/07 7574, WO-A 95/29386, WO-A 95/16897 or WO-A 99 40 394. Each of the therein illustrated measuring transducers comprises at least one essentially straight or curved measuring tube, which is accommodated in a measuring transducer housing and conveys or guides the—in given cases, also extremely rapidly, or extremely slowly—flowing fluid. In operation of the measuring system, the at least one measuring tube is caused to vibrate for the purpose of generating oscillation forms influenced by the fluid flowing through the measuring tube.
In the case of measuring transducers having two measuring tubes, these are most often integrated into the process line via a flow divider extending on the inlet-side between the measuring tubes and an inlet-side connecting flange, as well a via a flow divider extending on the outlet-side between the measuring tubes and an outlet-side connecting flange. In the case of measuring transducers having a single measuring tube, the latter communicates with the process line most often via an essentially straight connecting tube piece opening on the inlet-side, as well as via an essentially straight connecting tube piece opening on the outlet-side. Additionally, each of the illustrated measuring transducers having a single measuring tube comprises, in each case, at least one one-piece or multipart—for example, tube-, box- or plate-shaped—counteroscillator, which is coupled to the measuring tube on the inlet-side for forming a first coupling zone, and which is coupled to the measuring tube on the outlet-side for forming a second coupling zone, and which, during operation, essentially rests or oscillates opposite-equally to the measuring tube, thus with equal frequency and opposite phase. The inner part of the measuring transducer formed by means of the measuring tube and counteroscillator is most often held, especially in a manner enabling oscillations of the inner part relative to the measuring tube, in a protective measuring transducer housing alone by means of the two connecting tube pieces, via which the measuring tube communicates during operation with the process line. In the case of the measuring transducers—for example, as illustrated in U.S. Pat. Nos. 5,291,792, 5,796,010, 5,945,609, 7,077,014, US-A 2007/0119264, WO-A 01 02 816 or also WO-A 99 40 394—having a single, essentially straight, measuring tube, the latter and the counteroscillator are, as is quite usual in the case of conventional measuring transducers, oriented essentially coaxially relative to one another. In the case of usually marketed measuring transducers of the aforementioned type, the counteroscillator is also most often essentially tubular, and is embodied as an essentially straight, hollow cylinder, which is arranged in the measuring transducer in such a manner that the measuring tube is at least partially jacketed by the counteroscillator. Most often used as materials for such counteroscillators, especially also in the case of application of titanium, tantalum or zirconium for the measuring tube, are comparatively cost-effective steel types, such as, for instance, structural steel or free-machining steel.
Selected as the excited oscillation form—the so-called wanted mode—in the case of measuring transducers having curved, e.g. U, V- or Ω-like shaped, measuring tubes is usually that eigenoscillation form is selected, in the case of which the measuring tube moves in a pendulum-like manner at least partially in a lowest natural resonance frequency about an imaginary longitudinal axis of the measuring transducer, like a cantilever clamped on one end, whereby Coriolis forces are induced in the fluid flowing through dependent on the mass flow. These forces, in turn, lead to the fact that, superimposed on the excited oscillations of the wanted mode, in the case of curved measuring tubes and thus pendulum-like, cantilever oscillations, are bending oscillations of a frequency equal to the former according to at least one, likewise natural, second oscillation form, the so-called Coriolis mode. In the case of measuring transducers with a curved measuring tube, these cantilever oscillations in the Coriolis mode caused by Coriolis forces usually correspond to that eigenoscillation form, in the case of which the measuring tube also executes rotary oscillations about an imaginary vertical axis directed perpendicular to the longitudinal axis. In the case of measuring transducers with a straight measuring tube, in contrast, for the purpose of producing of mass flow dependent Coriolis forces, often such a wanted mode is selected, in the case of which the measuring tube executes at least partially bending oscillations essentially in a single imaginary plane of oscillation, such that the oscillations in the Coriolis mode are bending oscillations of equal oscillation frequency with the wanted mode oscillations and coplanar thereto. Due to the superpositioning of wanted- and Coriolis modes, the oscillations of the vibrating measuring tube registered by means of the sensor arrangement on the inlet-side and on the outlet-side have a measurable phase difference also dependent on the mass flow. Usually, the measuring tubes of such measuring transducers, applied e.g. in Coriolis mass flow meters, are excited during operation to an instantaneous natural resonance frequency of the oscillation form selected for the wanted mode, especially with an oscillation amplitude controlled to be constant. Since this resonance frequency is dependent, especially, also on the instantaneous density of the fluid, and the density of flowing fluids can, in addition to the mass flow, also be measured by means of market-usual Coriolis mass flow meters. Additionally, it is also possible, as, for example, is shown in U.S. Pat. Nos. 6,651,513 or 7,080,564, directly to measure the viscosity of the fluid flowing through by means of measuring transducers of the vibration type, for example, based on an exciter energy or excitation power required for maintaining the oscillations, and/or based on a damping of oscillations (especially those in the aforementioned wanted mode) of the at least one measuring tube resulting from a dissipation of oscillatory energy. Moreover, also other measured variables derived from the aforementioned primary measured values of mass flow rate, density and viscosity can be ascertained, such as, for instance, the Reynolds number; compare U.S. Pat. No. 6,513,393.
For exciting oscillations of the at least one measuring tube, measuring transducers of the vibration type have, additionally, an exciter mechanism driven during operation by an electrical exciter signal, e.g. a controlled electrical current, generated and correspondingly conditioned by the mentioned driver electronics. The exciter mechansim excites the measuring tube to bending oscillations in the wanted mode by means of at least one electro-mechanical, especially electro-dynamic, oscillation exciter acting practically directly on the measuring tube, and flowed through during operation by an electrical current. Furthermore, such measuring transducers comprise a sensor arrangement having oscillation sensors, especially electro-dynamic oscillation sensors, for the at least pointwise registering of inlet-side and outlet-side oscillations of the at least one measuring tube, especially those in the Coriolis mode, and for producing electrical sensor signals influenced by the process parameter to be registered, such as, for instance, the mass flow or the density, and serving as oscillation signals of the measuring transducer. As, for example, is described in U.S. Pat. No. 7,216,550, in the case of measuring transducers of the type being discussed, in given cases, also the oscillation exciter can at least at times be used as an oscillation sensor and/or an oscillation sensor can at least at times can be used as an oscillation exciter. The exciter mechanism of measuring transducers of the type being discussed usually includes at least one electrodynamic oscillation exciter and/or an oscillation exciter acting differentially on the at least one measuring tube and the in given cases present counteroscillator or the in given cases present other measuring tube, while the sensor arrangement comprises an inlet-side, most often likewise electrodynamic oscillation sensor, as well as at least one outlet-side oscillation sensor constructed essentially equally thereto. Such electrodynamic and/or differential oscillation exciters of usually marketed measuring transducers of the vibration type are formed by means of a magnet coil, through which an electrical current at least at times flows. In the case of measuring transducers having a measuring tube and a counteroscillator coupled thereto, the magnet coil is most often affixed to the latter. Such oscillation exciters further include a rather elongated, especially rod-shaped permanent magnet interacting with the at least one magnet coil, especially plunging into it, and serving as an armature and affixed correspondingly to the measuring tube to be moved. The permanent magnet and the magnet coil serving as an exciter coil are, in such case, usually oriented in such a manner that they extend essentially coaxially relative to one another. Additionally, in the case of conventional measuring transducers, the exciter mechanism is usually embodied in such a manner and placed in the measuring transducer in such a manner that it acts essentially centrally on the at least one measuring tube. In such case, the oscillation exciter (and, in this respect, the exciter mechanism) is—such as, for example, is also shown in the case of the measuring transducers proposed in U.S. Pat. Nos. 5,796,010, 6,840,109, 7,077,014 or 7,017,424—most often affixed from the outside at least pointwise along an imaginary central, peripheral line of the measuring tube. As an alternative to an exciter mechanism formed by means of oscillation exciters acting centrally and directly on the measuring tube—such as, among other things, is proposed in U.S. Pat. Nos. 6,557,422, 6,092,429 or 4,823,614—exciter mechanisms formed, for example, by means of two oscillation exciters affixed not in the center of the measuring tube, but instead rather at the inlet and outlet-sides thereof, respectively, can also be used, or—as is, among other things, proposed in U.S. Pat. Nos. 6,223,605 or 5,531,126—exciter mechanisms formed, for example, by means of an oscillation exciter acting between the in given cases present counteroscillator and the measuring transducer housing can also be used. In the case of most market-usual measuring transducers of the vibration type, the oscillation sensors of the sensor arrangement are, as already indicated, at least insofar as they work according to the same principle of action, embodied with essentially equal construction to the at least one oscillation exciter. Accordingly, also the oscillation sensors of such a sensor arrangement are most often in each case formed by means of 1) at least one magnet coil—usually affixed to the in given cases present counteroscillator—at least at times passed through by a variable magnetic field and, associated therewith, at least at times supplied with an induced measurement voltage, as well as by means of 2) a permanently magnetic armature, which delivers the magnetic field. The armature is affixed to the measuring tube, and interacts with the at least one coil. Each of the aforementioned coils is additionally connected by means of at least one pair of electrical connecting lines with the mentioned transmitter electronics of the in-line measuring device. The connecting lines are led most often on as short as possible paths from the coils via the counteroscillator to the measuring transducer housing.
As, among other things, is discussed in the previously mentioned U.S. Pat. Nos. 7,406,878, 7,305,892, 7,134,348, 6,513,393, 5,861,561, 5,359,881 or WO-A 2004/072588, a further parameter relevant for operation of the measuring system as such and/or for operation of the plant in which the measuring system is installed can be a pressure loss in the flow (for example, a pressure loss caused by the measuring transducer and, in this respect, by the measuring system) or a lowered pressure resulting therefrom in the outlet-side of the measuring transducer. Pressure loss in the flow is important, especially also for the case, in which the fluid has two or more phases, for instance, a liquid gas mixture, and/or in which, during operation, undesired cavitation (which can even endanger the structural integrity of the measuring transducer) as a result of subceeding, or falling beneath, a minimum static pressure in the flowing fluid must be prepared for, or unconditionally prevented.